Abstract: This paper studies the large deflection of cantilever beams. A simple method to obtain the deflection is developed through numerical and theoretical analyses. How to measure the extent of large deformation is discussed at the same time. The error between the accurate deflection and the approximate deflection, obtained by the approximate deformation equation, can be represented by the square of the ratio of the approximate deflection to the length of the beam. By using this relation, we can have an estimation of the accurate deflection value without complex calculation and numerical methods. This method can be used in practical applications.